In recent years, there have been high expectations for the realization of large-screen televisions with superior picture quality. One example of such televisions are televisions for the “HiVision” standard used in Japan. In the field of display devices, research is being performed into a variety of devices, such as CRTs (Cathode Ray Tubes), LCDs (Liquid Crystal Displays), and Plasma Display Panels (hereafter PDPs) with the aim of producing suitable televisions.
Cathode ray tubes that are conventionally used in televisions have superior resolution and picture quality. However, the depth and weight of CRT televisions increases with screen size, so that CRTs are not suited to the production of large televisions with screen sizes of forty inches or more. LCDs have some notable advantages, such as low power consumption and low driving voltages, but it is difficult to manufacture large-screen LCDs.
On the other hand, PDPs enable large-screen slimline televisions to be produced, with fifty-inch models already having been developed.
PDPs can be roughly divided into direct current (DC) types and alternating current (AC) types. At present, AC types, which are suited to the production of panels with fine cell structures, are prevalent.
A representative AC-type PDPs is described hereafter. Display electrodes are provided on a front cover plate. This cover plate is arranged in parallel with a back cover plate on which the address electrodes are provided, so that the sets of electrodes form a matrix. A gap left between the plates is partitioned by partition walls in the form of stripes. Layers of red, green, and blue phosphors are formed between the partition walls and discharge gas is sealed in these spaces. Driving circuits are used to apply voltages to the electrodes, which causes discharge and the emission of ultra-violet light. This ultra-violet light is absorbed by the particles of red, green and blue phosphors in the phosphor layers, which causes excited emission of light. This light forms an image on the panel.
Most PDPs of this type are manufactured by forming the partition walls on the back plate, forming the phosphor layers between these walls, and introducing the discharge gas after arranging the front cover plate on the back plate.
Japanese Laid-Open Patent Application No. H06-5205 teaches a commonly used method for forming the phosphor layers between the partition walls. In this method (a screen-printing method), the gaps between the partition walls are filled with phosphor paste which is then baked. However, it is difficult to produce a PDP with a fine cell structure using screen printing.
As one example, when producing a television that is fully compatible with the specification for Japanese “HiVision” broadcasts, screen resolution needs to be 1920 by 1125 pixels, so that the pitch (cell pitch) of the partition walls for a 42-inch screen is only around 0.1 to 0.15 mm and the gaps between partition walls are only around 0.08 to 0.1 mm wide. Since the phosphor inks used by screen-printing is highly viscose (generally in the region of tens of thousands of centipoise), it is difficult to apply the phosphor inks to the narrow gaps between partition walls accurately and at high speed. It is also difficult to produce the screen plates for a PDP of such a fine construction.
Aside from screen printing, phosphor layers can be formed using a photoresist film or ink-jet printing.
One example of a method that uses a photo-resist film is described in Japanese Laid-Open Patent Application No. H06-273925. In this method, resinous film that is sensitive to UV light and contain phosphors of the one of the three colors is placed between adjacent partition walls. Only parts of the resinous film that are used to form a phosphor layer of the desired color are exposed, and remaining parts are washed away. With this method, a film can be inserted between the partition walls with a fair degree of accuracy, even when the cell pitch is narrow.
However, for each of the three colors, a film has to be inserted, the desired parts of the film need to be exposed, and the remaining parts need to be washed away. This makes the manufacturing process difficult, with there being a further problem of the different colors often becoming mixed. Phosphors are a relatively expensive material and since the phosphors that are washed away are unsuited to recycling, this method is also costly.
Japanese Laid-Open Patent Application Nos. S53-79371 and H08-162019 teach techniques that use ink-jet printing. A liquid ink formed of phosphors and an organic binder is pressurized and so is expelled from a nozzle that scans an insulating board, thereby forming a desired pattern of phosphor ink on the surface. These ink-jet methods generally use phosphor inks that are manufactured in the following way. Phosphors are dispersed in a mixture including (1) an organic binder such as ethyl cellulose, acryl resin, or polyvinyl alcohol, (2) a solvent such as terpineol or butyl carbitol acetate using a disperser such as a paint shaker.
With this kind of ink jet method, ink can be accurately applied to the narrow channels between the partition walls, though the ink that is expelled from the nozzle tends to form droplets and so is only intermittently applied to the channels. As a result, it is difficult to apply ink smoothly along the stripe-like channels.
In Japanese Laid-Open Patent Application Nos. H08-245853 and H09-253749, the inventors of the present application describe a method where low-viscosity, highly fluid phosphor inks are used. These inks are pressurized and so are continuously expelled from a moving nozzle, thereby applying the inks smoothly.
However, if the phosphor inks have been applied in the above manner, blurred lines tend to appear along the partition walls and along the gaps in the address electrodes when the resulting PDP is driven. Such blurred lines are especially evident in areas of the screen where white is being displayed.
It is believed that such blurred lines appear due to inconsistencies in the phosphor layers formed in the channels or due to the mixing of different-colored phosphors. Inconsistencies appear in the phosphor layer for the reasons given below.    (1) During application, the phosphor ink becomes electrically charged, and so can be affected by electrical charge that builds up due to the manufacturing environment or conditions. This means that the amount of phosphor ink that is applied can vary at different positions on the PDP.    (2) If the phosphor inks of the three colors are applied one at a time in order, the phosphor inks for the second and third colors are applied with phosphor ink already present in the neighboring channels. Phosphor ink being applied is subject to rheological effects of the phosphor ink present in these neighboring channels, so that it is difficult to apply the ink evenly.
Note that if the phosphor ink of each color is allowed to dry properly before the next ink is applied, such rheological effects can be eradicated. However, the drying process has to be performed more often, making more equipment necessary and complicating the manufacturing process.    (3) When phosphor ink is applied in the channels between the partition walls, it is preferable for the nozzle to scan along the centers of the channels so as to apply the ink evenly. However, even if the nozzle moves in a straight line, inconsistencies in the width of the channels and curvature of the channels can prevent the nozzle from following the center of the channels, making the consistent application of ink extremely difficult. This problem is especially evident with PDPs that have a fine cell structure.    (4) If a highly fluid phosphor ink is applied using fine nozzle, the switching on and off of the nozzle is accompanied by variation in the amount of ink that is actually expelled from the nozzle and in the angle at which the ink jet emerges. This makes it difficult to accurately apply the phosphor ink between the partition walls.
As another problem, it is difficult to apply the phosphor ink to the side faces of the partition walls on both sides of the channels, so that the ink tends to accumulate at the base of the channels. A balanced application of phosphor ink to both the base and the side faces of the walls is therefore difficult to achieve. When the balance between the amounts of phosphor ink on the side faces of the walls and in the base is poor, high panel luminance is difficult to achieve.
The diameter of the nozzle used in inkjet methods needs to be small in keeping with the pitch of the partition walls. This makes it easy for the nozzle to become blocked and prevents the prolonged continuous application of phosphor ink. In particular, when making a highly intricate PDP with a partition wall pitch of 0.15 mm or below, the diameter of the nozzle has to be set at a narrower distance, making blockage of the nozzle more common.